1. Field of the Invention
The present invention is related to time signal generators and, more particularly, to a time signal generating system for generating a time signal from a broadcast signal provided by a geostationary navigation satellite.
2. Background Information
For various systems, such as triangulation systems or communications systems, it is desirable provide a time reference signal, such that the stations in the system are all operating in accordance with the same underlying time. In a triangulation system, for example, the stations must operate using the same time reference so that the position information from various stations can be combined to pinpoint the location of a transmitter.
Various mechanisms for producing time signals are known however, the mechanisms are relatively complex. In a GPS system, for example, a GPS receiver can determine a reference time using information transmitted by the GPS satellites. To obtain accurate timing information from the GPS satellite signals, however, the GPS receivers must compensate for multipath signals, doppler shifts, ionospheric delays, signal jamming, and so forth. Accordingly, the GPS receivers are relatively complex and often process both the L1 and L2 signals transmitted by the GPS satellites to determine accurate ionospheric delay information.
The invention is a time signal generation system that produces a reference time signal based on the signals transmitted by geostationary satellites, such as the satellites in the Wide Area Augmentation System (WAAS). A receiver in the system includes a directional antenna that is focused on the signals transmitted by the geostationary satellite. The antenna rejects multipath signals, and the receiver can thus track the satellite signal directly, without compensating for multipath distortion. Once the receiver is tracking the code and carrier, the receiver decodes the transmitted data to recover timing information, as well as satellite orbital correction information and an ionospheric model, or table, transmitted by the satellite. The receiver then uses the information to produce a corrected timing signal that drives a reference clock. In response, the reference clock produces a system time signal that is essentially as stable as the Cesium clock signals utilized by the WAAS system.